1. Field of the Invention
The present invention relates to the fabrication of turbine blade assemblies and, in particular, the mounting of turbine blades in a support member.
2. Prior Art
A typical turbine blade row is composed of a plurality of individual blades which are installed in a support member, the support member typically being an annular rotor, or hub.
A conventional turbine blade includes a root via which the blade is connected to the hub and blades may be mounted on the hub in a free-standing manner, in which each blade is supported only at its root, or the blades may be further connected together near their tips. In the art, free-standing construction is currently preferred.
When blades are assembled to a hub, particularly in the case of free-standing construction, movement of each blade relative to the hub in the tangential direction of the rotor must be eliminated to the greatest extent possible at low speeds. When a turbine is rotating at high speed, centrifugal forces act to stabilize the position of each blade relative to the hub. However, at . lower speeds, such as turning gear speeds, which are of the order of 3 rpm, there is a tendency for the blade roots to move circumferentially within their respective rotor grooves. Specifically, as the turbine assembly rotates points will be reached at which a blade is acted on by gravity such that the root shifts within its associated rotor groove. Such movement of the blade roots causes an effect known as "fretting", in which particles are worn from surfaces which rub together, and these particles then oxidize and harden, whereby they can seriously abrade the blade root and associated rotor groove bearing surfaces. Fretting thus reduces useful blade life in the root area due to metal fatigue.
For this reason, efforts have been made to prevent such relative movement between a blade root and its associated rotor groove at turning gear speeds.
It has previously been proposed to prevent such movements by interposing an adhesive or an expansible material in the interface between each blade root and its associated groove.
However, it has been found that adhesives which perform well from the standpoint of preventing relative movement between a blade and its associated rotor groove also leave a corrosive residue which is difficult to remove and damages the rotor groove and blade root surfaces. For these reasons, such adhesives are no longer considered acceptable.
Moreover, when it is desired to remove a blade which has been secured by means of an adhesive or expansible material, special steps must be taken, such as heating the material, to permit removal of a blade. Frequently, it is difficult to control the heating with sufficient precision, with the result that the material holding adjacent blades in place is partially degraded.
Heretofore, purely mechanical arrangements for securing a blade in place to prevent relative movement between each blade and its associated rotor have not been available.